1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices, and more particularly to an LCD device incorporating a cholesteric liquid crystal color filter (CCF) layer.
2. Discussion of the Related Art
Due to their light weight, thin profile, and low power consumption characteristics, LCD devices are currently being developed as next generation display devices. Generally, LCD devices are non-emissive type display devices capable of displaying images by exploiting anisotropic optical refractive index difference properties of liquid crystal material interposed between a thin film transistor (TFT) array substrate and a color filter (C/F) substrate. Among the various types of commonly used LCD devices, active matrix LCD (AM-LCD) devices are capable of displaying images at high resolution and are excellent at displaying moving images.
Since LCD devices are non-emissive, one type of LCD device, the transmissive LCD device, displays images using light emitted from an external light source (e.g., a backlight unit). The efficiency with which transmissive LCD devices transmit the light emitted from backlight units, however, is relatively low. For example, only about 7% of the light emitted from backlight units is actually transmitted by transmissive LCD devices. Therefore, backlight units of transmissive LCD must emit light at a relatively high intensity (brightness). Consequently, backlight units account for a relatively large percentage of all power consumed by transmissive LCD devices. Further, large capacity batteries must typically be used to supply a sufficient amount of power to the backlight unit. However, even when large capacity batteries are used, the operating times of transmissive LCD devices become limited.
To solve the aforementioned problems related to transmissive LCD devices, reflective LCD devices have been developed that do not require backlight units but, rather, use ambient light as a light source. A first type of reflective LCD device includes absorption type color filter layers and a reflective layer. A second type of reflective LCD device includes cholesteric liquid crystal color filter (CCF) layer for selectively reflecting and transmitting light. Accordingly, the CCF layer in the second type of reflective LCD device functions both as a color filter layer and as a reflective layer and enables the second type of reflective LCD device to display images having high color purity. Moreover, since a separate reflective layer is not necessary, processes used to fabricate the second type of reflective LCD devices are simplified compared to the first type of reflective LCD device.
Liquid crystal molecules within liquid crystal material exhibiting a nematic liquid crystal phase are regularly aligned along one direction. CCF layers are formed from multiple layers of cholesteric liquid crystal (CLC) material exhibiting the nematic liquid crystal phase wherein a rotation of liquid crystal molecules, and therefore reflectance characteristics, between the multiple layers of CLC material is different. The difference in reflectance characteristics allow colors to be selectively displayed by reflection and interference of the light. The rotation of liquid crystal molecules within CLC material generates a helical structure that may be defined by a direction of the molecular rotation and a pitch (e.g., the distance between liquid crystal molecules having the same alignment, measured along the axis of rotation) of liquid crystal molecules within the CLC material. The pitch of the CLC material is variable and determines the wavelength of light the CLC material reflects. The central wavelength of light reflected by the CLC material, λc, can be expressed as the product of the pitch, p, of the CLC material and the average refractive index, navg, of the CLC material (i.e., λc=navg·p). For example, if a pitch, p, of the CLC material is about 430 nm and an average refractive index of the CLC material is about 1.5, the central wavelength of the reflected light is about 650 nm and corresponds to the color red. CLC material capable of reflecting green and blue light can similarly be provided by forming CLC material to have the corresponding pitch.
FIG. 1 illustrates a cross-sectional view of a related art reflective liquid crystal display device incorporating a cholesteric liquid crystal color filter layer.
Referring to FIG. 1, first substrate 10 includes an inner surface that faces and is spaced apart from an inner surface of a second substrate 50. A light absorption layer 12 is formed on the inner surface of the first substrate 10 and a cholesteric liquid crystal color filter (CCF) layer 14 is formed on the light absorption layer 12 for selectively reflecting light having predetermined wavelength range. The light absorption layer 12 absorbs light of all wavelengths except for the light selectively reflected by the CCF layer 14. A first transparent electrode 16 is formed on the CCF layer 14 and a first orientation film 18 is formed on the first transparent electrode 16. An array element layer 52 is formed on the inner surface of the second substrate 50 and a second transparent electrode 54 is formed on the array element layer 52. A second orientation film 56 is formed on the second transparent electrode 54. A layer of liquid crystal material 70 is interposed between the first and second orientation films 18 and 56. A retardation film 60 is formed on an outer surface of the second substrate 50 and a polarizing plate 62 is formed on the retardation film 60.
Generally, a broadband quarter wave plate (QWP) having a retardation value of λ/4 or 3λ/4 is used as the retardation film 60 and changes a polarization state of light. For example, the broadband QWP converts circularly polarized light into linearly polarized light, and vice-versa.
Although not shown in FIG. 1, the array element layer 52 generally includes a plurality of gate lines, a plurality of data lines crossing the plurality of gate lines, and thin film transistors connected to respective ones of the gate and data lines at crossings of the gate and data lines. Pixel regions are defined by crossings of the gate and data lines.
FIG. 2A schematically illustrates optical driving principles of a related art reflective LCD device incorporating a CCF layer in the absence of a voltage applied to a layer of liquid crystal material. FIG. 2B schematically illustrates optical driving principles of a related art reflective LCD device incorporating a CCF layer in the presence of a voltage applied to a layer of liquid crystal material.
For convenience of illustration, the related art reflective LCD device shown in FIGS. 2A and 2B functions in a normally black mode (i.e., a black image is displayed in the absence of an applied voltage). Further, for convenience of illustration, only a red sub pixel region is shown in FIGS. 2A and 2B.
Referring to FIGS. 2A and 2B, the polarizing plate 62 is provided as a linear polarizer having a polarization axis of 0° and the retardation film 60 is provided as a broadband quarter wave plate (QWP) capable of altering the phase of incident light by +45° and the phase of reflected light by −45°. In the absence of an applied voltage, the layer of liquid crystal material 70 has a first retardation value of λ/2 and, in the presence of an applied voltage, the layer of liquid crystal material 70 has a second retardation value of 0. The layer of liquid crystal material 70, first orientation film 18, and second orientation film 56 (from FIG. 1) constitute a parallel cell and rubbing directions of the first and second orientation films 18 and 56 cross each other at an angle of 180°. The CCF layer 14 selectively reflects only left-handed circularly polarized light having a wavelength corresponding to the color red.
Referring now to FIG. 2A, non-polarized ambient light incident to the polarizing plate 62 becomes linearly polarized light in correspondence with the polarization axis of the polarizing plate 62. Thus, linearly polarized light having a polarizing angle of 0° is transmitted by the polarizing plate 62 and is subsequently converted into left-handed circularly polarized light by the retardation film 60. Since the layer of liquid crystal material 70 has the first retardation value of λ/2 in the absence of an applied voltage (i.e., V=0; off state), the left-handed circularly polarized light transmitted by the retardation film is converted into right-handed circularly polarized light by the layer of liquid crystal material 70. Further, since the CCF layer 14 selectively reflects only left-handed circularly polarized light having a wavelength range corresponding to the color red, the right-handed circularly polarized light is transmitted by the CCF layer 14 and is absorbed by the light absorption layer 12. Accordingly, the reflective LCD device is maintained in a black state.
Referring now to FIG. 2B, non-polarized ambient light incident to the polarizing plate 62 becomes linearly polarized light in correspondence with the polarization axis of the polarizing plate 62. Thus, linearly polarized light having a polarizing angle of 0°, transmitted by the polarizing plate 62, is subsequently converted into left-handed circularly polarized light by the retardation film 60. Since the layer of liquid crystal material 70 has the second retardation value of 0 in the presence of the applied voltage of (i.e., V=V0, wherein V0 is the turn-on voltage of the layer of liquid crystal material 70), the left-handed circularly polarized light transmitted by the retardation film is not converted by the layer of liquid crystal material 70. Further, since the CCF layer 14 selectively reflects only left-handed circularly polarized light having a wavelength range corresponding to the color red, red left-handed circularly polarized light transmitted by the layer of liquid crystal material 70 is reflected by the CCF layer 14. The reflected red left-handed circularly polarized light is then transmitted by the layer of liquid crystal material 70 and is subsequently converted into red linearly polarized light having a polarizing angle of 0° by the retardation film 60. As the red linearly polarized light transmitted by the retardation film 60 has the polarizing angle of 0°, the red linearly polarized light is transmitted by the polarizing plate 62 having the polarizing angle of 0°. The optical driving principles described above with respect to red light are similarly applicable to wavelength ranges of light corresponding to green and blue colors. Accordingly, the related art reflective LCD device maintains a white state by combining the reflected red, green, and blue light.
The related art reflective LCD device shown in FIGS. 1, 2A and 2B can also be fabricated as a transmissive LCD device incorporating the CCF layer. Accordingly, CCF layers capable of selectively reflecting wavelength ranges of light corresponding to green and blue colors are formed within the red sub pixel region such that only red light is transmitted by the CCF layers.
FIG. 3 illustrates a cross-sectional view of a related art transmissive LCD device incorporating a CCF layer.
Referring to FIG. 3, first substrate 110 includes an inner surface that faces and is spaced apart from an inner surface of a second substrate 150. A cholesteric liquid crystal color filter (CCF) layer 112, including first and second sub-CCF layers 112a and 112b, respectively, is formed on the inner surface of the first substrate 110. A first transparent electrode 114 is formed on the CCF layer 112 and a first orientation film 116 is formed on the first transparent electrode 114. A first polarizing plate 120 is formed on an outer surface of the first substrate 110. An array element layer 152 is formed on an inner surface of the second substrate 150 and a second transparent electrode 154 is formed on the array element layer 152. A second orientation film 156 is formed on the second transparent electrode 154. A retardation film 160 is formed on an outer surface of the second substrate 150 and a second polarizing plate 162 is formed on the retardation film 160. The first polarizing plate 120 is formed of a cholesteric liquid crystal (CLC) material that selectively reflects left-handed or right-handed circularly polarized light of all wavelengths (e.g., light of all colors). The CCF layer 112 selectively reflects left-handed or right-handed circularly polarized light within a predetermined wavelength range (i.e., light of a predetermined color). Accordingly, the first polarizing plate 120 and the CCF layer 112 are typically made of different materials.
A layer of liquid crystal material 170 is interposed between the first and second orientation films 116 and 156. The layer of liquid crystal material 170, first orientation film 116, and second orientation film 156 constitute a parallel cell an rubbing directions of the first and second orientation films 116 and 156 cross each other at an angle of 180°. A backlight unit 180 is disposed beneath the first polarizing plate 120.
Although not shown in FIG. 3, the array element layer 152 generally includes a plurality of gate lines, a plurality of data lines crossing the gate lines, and thin film transistors connected to respective ones of the gate and data lines at crossings of the gate and data lines. Pixel regions are defined by crossings of the gate and data lines.
FIG. 4A schematically illustrates optical driving principles of a related art transmissive LCD device incorporating a CCF layer in the absence of a voltage applied to a layer of liquid crystal material. FIG. 4B schematically illustrates optical driving principles of a related art transmissive LCD device incorporating a CCF layer in the presence of a voltage applied to a layer of liquid crystal material.
For convenience of illustration, the related art transmissive LCD device of FIGS. 4A and 4B functions in a normally black mode (i.e., a black image is displayed in the absence of an applied voltage). Further, for convenience of illustration, only a red sub pixel region is shown in FIGS. 4A and 4B.
Referring to FIGS. 4A and 4B, the first polarizing plate 120 is formed of cholesteric liquid crystal (CLC) material that selectively reflects only right-handed circularly polarized light of all wavelengths (e.g., all colors). The second polarizing plate 162 is provided as a linear polarizer having a polarization axis of 0°. The retardation film 160 is provided as a broadband quarter wave plate (QWP) capable of altering the phase of incident light by +45° and the phase of reflected light by −45°. The cholesteric liquid crystal color filter (CCF) layer 112 includes first and second sub-CCF layers 112a and 112b that selectively reflect left-handed circularly polarized light having wavelengths within predetermined ranges corresponding to green and blue colors, respectively. In the absence of an applied voltage, the layer of liquid crystal material 170 has a first retardation value of λ/2 and, in the presence of an applied voltage, the layer of liquid crystal material 170 has a second retardation value of 0.
Referring now to FIG. 4A, of the non-polarized light emitted by the backlight unit 180 and incident to the first polarizing plate 120, right-handed circularly polarized light of all wavelengths is selectively reflected and only left-handed circularly polarized light of all wavelengths is transmitted by the first polarizing plate 120. The CCF layer 112, including the first and second sub CCF layers 112a and 112b, then selectively reflects only the incident left-handed circularly polarized light having wavelength ranges corresponding to green and blue colors. Accordingly, only left-handed circularly polarized light having a wavelength range corresponding to the color red is transmitted by the CCF layer 112. Since the layer of liquid crystal material 170 has the first retardation value of λ/2 in the absence of an applied voltage (i.e., V=0; off state), the left-handed circularly polarized light transmitted by the CCF layer 112 is converted into right-handed circularly polarized by the layer of liquid crystal material 170. The right-handed circularly polarized light is then converted into linearly polarized light having a polarizing angle of 90° via the retardation film 160. Since the second polarizing plate 162 is a linear polarizer having a polarization axis of 0°, the linearly polarized light having a polarizing angle of 90° does not pass through the second polarizing plate 162. Accordingly, the transmissive LCD device is maintained in a black state.
Referring now to FIG. 4B, of the non-polarized light emitted by the backlight unit 180 and incident to the first polarizing plate 120, right-handed circularly polarized light of all wavelengths is selectively reflected and only left-handed circularly polarized light of all wavelengths is transmitted by the first polarizing plate 120. The CCF layer 112, including the first and second sub CCF layers 112a and 112b, then selectively reflects only the incident left-handed circularly polarized light having wavelength ranges corresponding to green and blue colors. Accordingly, only left-handed circularly polarized light having a wavelength range corresponding to the color red is transmitted by the CCF layer 112. Since the layer of liquid crystal material 170 has the second retardation value of 0 in the presence of the applied voltage (i.e., V=V0; on state), the left-handed circularly polarized light transmitted by the CCF layer 112 is also transmitted by the layer of liquid crystal material 170. The left-handed circularly polarized light is then converted into linearly polarized light having a polarizing angle of 0° via the retardation film 160. Since the second polarizing plate 162 is a linear polarizer having a polarization axis of 0°, the linearly polarized light having a polarizing angle of 0° is transmitted by the second polarizing plate 162. The optical driving principles described above with respect to red light are similarly applicable to wavelengths of light corresponding to green and blue colors. Accordingly, the related art transmissive LCD device maintains a white state by combining the transmitted red, green, and blue light.
As mentioned above, the related art LCD devices incorporating the CCF layer use retardation films formed from a broadband QWP to display images. The broadband QWP compensates phase differences for broadband wavelengths light (e.g., light of having wavelength ranges corresponding to red, green, and blue colors). Related art broadband QWPs generally are constructed of a multi-layer system including a conventional QWP and a half wave plate (HWP). Accordingly, relatively large fabrication costs are associated with broadband QWPs and the reliability of LCD devices including a broadband QWP becomes reduced due to shrinkage and distortion problems inherent to broadband QWPs.